Work that is in progress and proposed for continuation this year includes the role of temperature and relative humidity in photosensitivity reactions, as well as the role of biologic factors of the host resulting in endogenously produced photosensitizers such as porphyrins. Although exogenous photosensitizing substances are well recognized causes of occupational dermatoses due to their primary irritant ("phototoxic") or allergic ("photoallergic") properties, little is known regarding the mechanisms of their photosensitivity properties on a molecular and submolecular biologic level. Our studies this year will extend into this field particularly stressing protein binding sites. Red blood cell studies in photohemolysis will be extended from erythropoietic protoporphyria to include lead poisoning. A new research goal for the coming year is to quantitate plasma levels of protoporphyrin in both conditions. In addition to the use of in vitro models and in vivo animal experimentation, our studies will continue to center on the recognition of new adverse photosensitivity reactions in man due to the various compounds which are being introduced into our environment by new developments in industry, manufacturing, and medical practice. BIBLIOGRAPHIC REFERENCES: Piomelli, S., Lamola, A., Poh-Fitzpatrick, M., Seaman, C. Herber, L.C.: Erythropoietic Protoporphyria and Pb Intoxication: The Molecular Basis for Difference in Cutaneous Photosensitivity. I. Different Rates of Diffusion of Protoporphyrian from the Erythrocytes, Both in vivo and in vitro. (Accepted Journal of Clinical Investigation, April, 1975). Harber, L.C., Lamola, A., Piomelli, S., Poh-Fitzpatrick, M. Yamane, T.: Erythropoietic Protoporphyria and Pb Intoxication. The Molecular Basis for Difference in Cutaneous Photosensitivity. II. Different Binding of Erythrocyte Protoporphyrin to Hemoglobin. (Accepted Journal of Clinical Investigation April, 1975).